of course traders must learn from the mistakes they have made, it is very difficult, but it can make you more confident, and indeed it takes time to get up, if you don't want to lose your money too much, use small capital, your emotions will surely will be controlled 
Some mistakes will happen again if we don't take it seriously and sometimes a small mistakes/decisions will lead us to a big problem. Confident on trading will come to you if we already knew every detail, strategies and analyze in the markets. There are many strategies you can do to make profit in trading and all you have to do is find the best strategy that you can easily do then find other strategy that can may help you more in other trading. I believe that everything will be in proper order in the right time of trading. We must trust the process of learning every day. However OP was referring more to inner confidence than to real technical knowledge - which I do not support.
I claim over confident traders as petty tyrant, because you have to be careful about your own money